When manufacturing, for example, a semiconductor device or a flat panel display, a substrate such as a semiconductor wafer or a liquid crystal substrate has been processed in the respective manufacturing processes using a substrate processing apparatus.
In the substrate processing apparatus, a substrate is transferred while being held by a substrate holding unit, or various processes are performed such as cleaning, etching or applying of a resist while rotating the substrate held by the substrate holding unit.
In the substrate processing apparatus of the related art, in order to detect whether a substrate is properly held by a substrate holding unit, laser light is irradiated from a light projecting device onto a surface of the substrate held in the substrate holding unit and an amount of light reflected from the surface of the substrate is detected by a light receiving device to determine whether the substrate is properly held by the substrate holding unit based on the detected amount of light. See, for example, Japanese Patent Application Laid-Open Publication No. 2003-229403.
However, in the substrate processing apparatus of the related art, even though the substrate is properly held by the substrate holding unit, the detected value by the light receiving device becomes lower when reflectivity of the substrate surface is low due to, for example, an influence by material or surface (reflecting surface) condition of a substrate.
Therefore, in the substrate processing apparatus of the related art, even though the substrate is properly held by the substrate holding unit, when the amount of light detected by the light receiving device is lower than the threshold value due to, for example, the influence by material or surface condition of a substrate, there is a concern for erroneous determination that the substrate is not properly held.